The human immunodeficiency virus (HIV-1), the etiologic agent of acquired immunodeficiency syndrome (AIDS), encodes a number of regulatory proteins. One of them, called Tat, is required primarily to increase the rate of transcription from the viral long terminal repeat (LTR) during viral growth and may also regulate expression of cellular genes and mediate apoptosis. Tat can be taken up by HeLa cells and localizes rapidly to the nucleus.
Despite extensive study, the mechanism of Tat-responsive action is still poorly understood. In most cellular assay systems and the cell free systems, the major effects of Tat are on transcription elongation, but effects on the rate of initiation have also been reported. Transactivation by Tat is critically dependent upon upstream Sp1 binding sites in the DNA and on an RNA sequence, called TAR, which is encoded by DNA sequences downstream of the site of transcription initiation. The Sp1-binding sites interact with the cellular transactivator Sp1 that cooperates with Tat, bound to TAR, to stimulate transcription. Tat can also function when bound to DNA as a GAL4 fusion protein. Genetic and biochemical studies have suggested that a human-specific factor acts as a Tat cofactor to facilitate binding of Tat to TAR and mediates transcriptional activation. This putative cofactor is probably encoded by a gene on human chromosome 12 because human chromosome 12 greatly stimulates Tat-responsive activation in rodent cells. Furthermore, high levels of the amino-terminal portion of Tat can squelch transactivation by Tat and chimeric Tat proteins in vivo and in vitro, suggesting that at least one coactivator may interact with the amino-terminal portion of Tat. A partially purified cofactor can support Tat-activated transcription in an in vitro transcription system containing Sp1, highly purified general transcription factors and RNA polymerase II. Several candidate coactivators, including TBP1 (Nelbock et al., 1990, Science 248:1650) have been identified. Although these proteins have some of the expected characteristics of a coactivator, none has been determined to interact directly with the amino-terminal portion of Tat and to participate directly in the action of Tat as a cofactor. A cellular acidic activator, Tat-associated protein (TAP) has been shown to interact directly with the core domain of Tat and the general transcription factor TFIIB (Yu et al., 1995, J. Virol. 69:3007). However, since TAP is conserved among eukaryotes and is expressed in human and rodent cells, it can be excluded as a species-specific cofactor for Tat.
In order to develop effective therapies for the treatment and prevention of AIDS, elucidation of the mechanisms by which it replicates in the human host is essential. Therefore, in view of the aforementioned deficiencies attendant to this mechanism, it is apparent that there still exists a need in the art for the identification and characterization of the cofactors which participate in the HIV-1 transcription process.